The heart is the center of a person's circulatory system. The left portions of the heart, including the left atrium and left ventricle, draw oxygenated blood from the lungs and pump it to the organs of the body to provide the organs with their metabolic needs for oxygen. The right portions of the heart, including the right atrium and right ventricle, draw deoxygenated blood from the body organs and pump it to the lungs where the blood gets oxygenated. These mechanical pumping functions are accomplished by contractions of the heart. In a normal heart, the sinoatrial node, the heart's natural pacemaker, generates electrical impulses that travel through normal electrical conduction pathways to cause the atria, and then the ventricles, to contract.
Tachyarrhythmia occurs when electrical impulses are generated from one or more abnormal electrical foci and/or when abnormal electrical conduction loops are formed in the heart. When tachyarrhythmia occurs, the heart contracts at a rate higher than a normal heart rate. When the heart rate rises beyond a certain point, the atria and/or the ventricles do not have sufficient time to be filled with blood before each contraction occurs. Consequently, the heart fails to pump sufficient blood to the body organs to meet their metabolic demand. Tachycardia, such as atrial flutter (AFL) or ventricular tachycardia (VT), occurs when the heart contracts at a tachyarrhythmia rate with a substantially regular rhythm. Fibrillation, such as atrial fibrillation (AF) or ventricular fibrillation (VF), occurs when the heart contracts at a tachyarrhythmia rate with a substantially irregular rhythm. VF is a life threatening condition requiring immediate medical treatment such as ventricular defibrillation. AF, though not directly life threatening, also needs medical treatment such as atrial defibrillation to restore a normal cardiac function and to prevent the deterioration of the heart.
Implantable medical devices such as implantable cardioverter/defibrillators (ICDs) are used to treat tachyarrhythmias, including tachycardia and fibrillation. ICDs are capable of delivering defibrillation shock pulses. Various types of ICD are also capable of delivering pacing pulses. Because a defibrillation shock pulse carries a substantial amount of energy and causes substantial discomfort to the patient, it is to be used only when considered necessary. For example, a known type ICD delivers anti-tachycardia pacing (ATP) therapy in response to a detection of tachycardia. Only if the ATP therapy fails to restore a normal cardiac rhythm, a defibrillation shock pulse is delivered. In response to a detection of fibrillation, on the other hand, a defibrillation shock pulse is delivered without delivering the ATP therapy.
Thus, to determine a suitable type of therapy in response to a detection of tachyarrhythmia, there is a need to classify that tachyarrhythmia, such as by discriminating between tachycardia and fibrillation.